Mounting electronic components

ABSTRACT

A method of mounting an electronic component having at least one contact extending across a part of its undersurface may include providing a support smaller in area than the undersurface of the component and having a contact pad for connection to the contact. The contact pad may have a first portion extending across an upper surface of the support adjacent one edge and a second portion extending from the edge across a side surface of the support. The method may also include positioning the electronic component and the support with the undersurface of the component adjacent the upper surface of the support. This is done so that the first portion of the contact pad is aligned with and spaced apart from a first portion of the contact, and the second portion of the contact pad is aligned with and disposed inwardly of a second portion of the contact.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of electronic circuits, and, more particularly, to a method of mounting electronic components. Further, the present invention is particularly applicable to the mounting of integrated circuits (ICs) on printed circuit boards (PCBs), including flexible printed circuit boards, though the present invention may also be used to mount other components on PCBs, or to mount components on other types of carriers, for example.

BACKGROUND OF THE INVENTION

[0002] There are many situations where it is desirable to minimize the overall dimensions of an electronic device. One example is a miniature video camera using an image sensor formed as a single IC.

[0003] An IC is generally mounted on a PCB having a larger area. Such an arrangement is illustrated in FIG. 1 in a surface mount form. An IC 10 has a plurality of contacts 12 extending down the side face 14 and under the undersurface 16 of the IC 10. The IC 10 is mounted on a PCB 18 which has a plurality of copper contact pads 20. The contact 12 is connected to the contact pad 20 by a body 22 of solder which includes a planar portion 22A and a fillet 22B. This arrangement requires the PCB 18 to extend beyond the side of the IC 10 by a distance of at least A. In practice, a greater distance B will be required, since the copper contact pad 20 will not normally extend to the edge of the PCB 18.

[0004] Another prior art approach includes mounting an IC in a manner which reduces the horizontal dimension from that of FIG. 1. This prior art approach is illustrated in FIG. 2, in which like parts are denoted by like reference numerals. Here, the IC 10 and the PCB 18 are of the same horizontal extent. The contact 12 is positioned only on the underside 16 of the IC 10, and the copper contact pad 20 extends to the edge of the PCB 18. The connection is made by a solder body 22′ which has only a planar portion. The solder fillet is lost, which degrades the mechanical strength and reliability of the connection, and may provide a poorer electrical connection.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is to provide a method of mounting electronic components which facilitates minimization of the size of a device.

[0006] The invention in its various aspects is defined in the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] An embodiment of the invention will now be described, by way of example, referring to the drawings, in which:

[0008]FIGS. 1 and 2, previously discussed, are cross-sectional views of various prior art mounting configurations;

[0009]FIG. 3 is a cross-sectional view illustrating one embodiment of the present invention;

[0010]FIG. 4 is a cross-sectional view illustrating an alternate embodiment of the present invention; and

[0011]FIGS. 5A and 5B are perspective views showing the manufacture of a circuit board for use in the embodiments of FIGS. 3 and 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Referring to FIG. 3, an IC 10 has a plurality of contacts 12 (only one of which is shown for clarity of illustration) extending down the side face 14 and under the undersurface 16 of the IC 10. The IC 10 is mounted on a PCB 18, which may be a flexible PCB and which has a plurality of copper contact pads 20. Again, only one of the copper contact pads 20 is shown for clarity of illustration. The copper contact pad extends over the top surface 24 and down the side 26 of the PCB 18. The contact pad 20 is typically connected to other circuitry (not shown) by a copper track 28.

[0013] The PCB 18 is smaller in horizontal dimension than the IC 10, its edge being set in by a distance X. The contact 12 is connected to the contact pad 20 by a body 22 of solder which includes a planar portion 22A and a fillet 22C engaging the side of the contact pad 20. The embodiment of FIG. 3 thus provides the advantages of a filleted solder joint within a smaller package size. A modified form of the embodiment of FIG. 3 is illustrated in FIG. 4, in which the contact 12 is located only on the under surface 16 of the IC 10.

[0014] Turning now to FIG. 5, one method for forming the contact pad 20 used in the foregoing embodiments will now be described. A PCB substrate 28 is drilled at 30 and is then plated in the conventional manner to form a track 32 and a plated-through via 34. The board is then cut along the line Y-Y to produce the side contact area 36 seen in FIG. 5B. The mounting of the IC 10 to the PCB 18 can be carried out using conventional techniques, preferably those in which solder paste is deposited by printing techniques and the assembly is heated to reflow the solder.

[0015] Yet another feature of the invention includes a modification of the conventional process. It is conventional to deposit solder paste in the required pattern on the PCB, press the IC or other device(s) into the solder paste from above, and then heat the assembly to reflow the solder. This procedure may be followed with the present invention, but can suffer difficulties because the PCB is smaller than the IC. Thus, according to the present invention, the IC may be positioned upside down, solder paste deposited in the required pattern on the underside of the IC, and the PCB pressed in an upside down orientation onto the solder paste. Further, this orientation is maintained during heating, solder reflow, and cooling.

[0016] Those of skill in the art will appreciate that various modifications and improvements may be made to the foregoing embodiments within the scope of the present invention. 

That which is claimed is:
 1. A method of mounting an electronic component which has at least one contact extending across a part of the undersurface of the electronic component, the method comprising: providing a support which is smaller in area than the undersurface of the electronic component, the support having a contact pad for connection to said contact, the contact pad having a first portion extending across an upper surface of the support adjacent on edge thereof and a second portion extending from said edge across a side surface of the support; positioning the electronic component and the support with the under surface of the electronic component adjacent the upper surface of the support such that the first portion of the contact pad is aligned with and spaced from a first portion of the contact and the second portion of the contact pad is aligned with and disposed inwardly of a second portion of the contact; and forming a solder connection between the contact and the contact pad, the solder connection comprising a generally planar portion between the first portions of the contact and the contact pad, and a fillet portion extending angularly from the second portion of the contact to the second portion of the contact pad.
 2. The method of claim 1, in which the solder connection is formed by placing solder paste between the contact and the contact pad, and heating the assembly to cause flow of the solder paste.
 3. The method of claim 2, in which the electronic component is held upside down, the solder paste is applied to the electronic component, the support is applied upside down to the solder paste, and the assembly is heated.
 4. The method of claim 3, in which the assembly is maintained upside down until it is cooled.
 5. The method of any preceding claim, in which the electronic component is an integrated circuit having a plurality of contacts, at least part of each contact extending across a part of the under surface of the integrated circuit; and the support is a printed circuit board which includes a contact portion for each of said contacts.
 6. The combination of an integrated circuit and a printed circuit board, in which the printed circuit board is smaller in area than the integrated circuit.
 7. The combination of claim 6, in which the printed circuit board has a number of contact pads which each have a first portion extending across an upper surface of the printed circuit board adjacent on edge thereof and a second portion extending from said edge across a side surface of the printed circuit board; the integrated circuit has a corresponding number of contacts each of which has at least a part thereof extending across part of the undersurface of the integrated circuit; and each contact is connected to a respective contact pad by a solder connection which includes a fillet portion.
 8. A printed circuit board for use in the combination of claim 6 or claim 7, the printed circuit board including contacts each of which has a portion extending across a side face of the printed circuit board.
 9. A method for making the printed circuit board of claim 8, the method including the steps of forming one or more plated through-holes, and cutting the board along a path which passes through said hole(s). 